1. Technical Field
This invention relates to controlling a pilot signal in Digital Television (DTV) transmission to better track symbol timing and aiding in pilot signal removal prior to direct use of QAM (Quadrature Amplitude Modulation) demodulator equipment.
2. Background Art
Digital television (DTV) signals in the USA are broadcast using the Advanced Television Standard Committee (ATSC) television standard modulation system which is an eight level vestigial sideband (8-VSB) modulation with a suppressed carrier signal. Conventional 8-VSB receiver designs complex demodulate the received signal with a pilot tone on zero frequency. Under ideal channel conditions, this allows the data symbols to stream on only one of two complex demodulated channels (known as xe2x80x9cI-channel onlyxe2x80x9d processing).
The MPEG-2 (Motion Picture Experts Group) packets of the 8-VSB spectrum comprise 208 bytes or 1664 bits corresponding to 832 symbols. There are eight symbol levels (xe2x88x927, xe2x88x925, xe2x88x923, xe2x88x921, 1, 3, 5, 7).
Currently published 8-VSB transmitter designs recommend applying a DC offset to the data symbol values prior to root raised cosine filtering. The eight symbol levels are combined with a 1.25 positive offset added to create a DC component, pilot tone.
A pilot tone is included in the 8-VSB signal design. The symbol values entering a zero-ISI VSB filter are offset by a DC value to generate the pilot tone. This approach causes the pilot tone to have a particular phase with respect to the symbol detection timing. Present receiver implementations may suggest that this phase is of no consequence when I-channel only receiver processing is conducted, because the pilot tone is used for frequency acquisition only. A complex filter is used to generate the I-channel.
Recent discoveries, as described in the aforementioned copending U.S. patent application Ser. No. 09/422,446 filed on Oct. 21, 1999 and issued at U.S. Pat. No. 6,292,518 entitled xe2x80x9cUse of 64-QAM circuitry for receiving and decoding 8-VSB signalsxe2x80x9d, have revealed that the 8-VSB transmission may be received using quadrature amplitude modulated (QAM)-based receiver structures. This is accomplished by using a raised cosine filter convolved with a complex exponential at 2.69 MHZ.
C(t)=h(t)xc3x97cos(xcfx80n/16)+jh(t)xc3x97cos(xcfx80n/16),
where xcfx80n/16=(2xcfx802.69n/(8xc3x9710.76)). Using QAM-based receiver designs, superior performance is obtained over the I-channel only receiver designs recommended in the Guide to the Use of the ATSC Standard.
When inserting the pilot tone, as recommended by the Guide to the Use of the ATSC Standard, the phase of the pilot tone is zero relative to the I-channel, and the pilot tone maximally interferes with the symbol detector in a QAM-based receiver design. The pilot phase at 0xc2x0 moves the symbol threshold values. Multipath effects will dynamically vary the pilot tone amplitude and make successful estimates of the symbol value at the timing instant difficult.
In order to use a QAM demodulator for directly demodulating 8-VSB DTV signals, it is necessary for the receiver chain to first effectively remove the pilot tone which is specified by the DTV standard in order to better enable frequency synchronization. If the phase of the pilot tone is unspecified, the problem becomes one of first estimating and actively removing a tone of arbitrary phase before using the QAM demodulator. This incurs added complexity and cost.
An ATSC-compliant pilot tone is generated within an 8-VSB transmission in order to enable QAM receiver designs to more efficiently process the transmitted/received 8-VSB signal. The method and apparatus also efficiently removes the pilot tone before directly using conventional QAM demodulator equipment.